An electric propulsion system for a traction vehicle, such as a large haulage truck, typically comprises a prime mover-driven electric generating means for supplying electric power to a pair of high horsepower electric traction motors respectively connected in driving relationship to a pair of wheels on opposite sides of the vehicle. The prime mover is commonly a diesel engine, and the traction motors are generally adjustable speed, reversible direct current (DC) electric motors. A vehicle operator controls the vehicle speed and direction of travel, i.e., forward or reverse, by manipulation of a speed control pedal and a forward-reverse selector switch. This speed control pedal is adapted to control the engine speed (rpm) which determines the power output of the generating means, thus varying the magnitude of the voltage applied to the traction motors.
Deceleration of a moving vehicle is accomplished by releasing the speed control pedal and either allowing the vehicle to coast or activating its mechanical or electrical braking system. In the electric braking mode of operation, the motors behave as generators, and the magnitude of the voltage generated across the armature windings of each motor is proportional to the rotational speed and the field excitation current of the motor. Dynamic braking resistor grids are connected across the armatures of the respective motors to dissipate the electric power output of the motors during electric braking. The average magnitude of current in each resistor grid is a measure of the braking effort of the associated motor.
During electrical braking, it is common in such systems to have the braking resistor grids connected in parallel circuit arrangement with the generating means. The generating means includes a controlled rectifier bridge circuit for coupling power from an alternator driven by the prime mover. The bridge circuit converts the AC power from the alternator to DC power for the electric motors. During regenerative electrical braking, the controlled rectifiers are controlled in a manner to convert the DC power generated by the electric motors into suitable AC power for driving the alternator as a motor. Retarding energy is dissipated in the braking resistor grids and by feeding energy into the alternator for driving the prime mover. The prime mover is generally connected not only to drive the alternator but to also power various vehicle accessories. During regenerative braking, retard energy drives the prime mover and powers the vehicle accessories resulting in a fuel savings.
The magnitude of available braking power during retard is the product of motor voltage and current. In order to maximize the retard effort of the electric motors, it is desirable to allow motor armature voltage to rise to a value sufficient to generate maximum safe motor current, generally restricted to the motor commutation limit. This voltage, at higher motor speeds, is greater than can be safely applied to the controlled rectifier bridge circuit requiring that the circuit be disabled at such higher motor speeds and thereby preventing the use of regenerative braking at higher speeds. It would therefore be desirable to provide a method and apparatus for allowing use of regenerative braking at higher motor speeds and reducing the use of dynamic braking resistor grids.
In control systems for the above described electric motors as well as others, it is common to use semiconductor controlled rectifiers (SCR) in the bridge circuit interconnecting the alternator and motor. One such bridge circuit may be used between an alternator primary winding and a motor armature while another bridge circuit interconnects an alternator tertiary winding and a motor field winding. The SCR's are gated or switched between conductive and non-conductive states in a well known manner in order to control the magnitude of current in the motor armature and field windings for regulating motor torque and polarity, i.e., forward and reverse rotation in propulsion and retard effort in electrical braking. In operation, the SCR's are switched such that all motor load current passes through the associated alternator winding. If the circuit can be operated in a manner to reduce the amount of load current through the alternator winding, the AC source rating can be reduced at a cost saving. Furthermore, the requirements for a smoothing reactor can also be reduced at a further savings.